While it is generally true for many flowering trees that when they are in strife they tend to flower heavily, we doubt this is true for pohutukawa. The species may respond to environmental stress however so that the year following a drought for example heavy flowering may occur while following a moist good growing year it may crack on the vegetative growth. No-one has shown what it is that determines whether a tree sets a vegetative bud or a flower bud, but you can bet it is an environmental trigger.
Our native beeches (Nothofagus) have similar complex mechanisms which lead to a mast (heavy flowering) year about every 7 years, which may also be drought related. So in short heavy flowering by a pohutukawa tree probably indicates that the next year is likely to be focused on vegetative growth.
Pohutukawa is amazingly resilient. It can take an enormous amount of abuse and still bounce back. The very big and old trees are the greatest at risk as the wood to foliage ratio is high. This means that there is lots of wood supported by limited foliage. Young trees have a low wood to foliage ratio – a lot more foliage relative to wood. When an old tree is in danger of dying from say possum defoliation, the ultimate solution is to cut it off about 2m from the ground. They usually sprout like crazy and the wood to foliage ratio is rapidly reversed and they grow like teenagers again. Before taking this action however, please of course contact your local council to find out the rules for carrying out work on trees in your area.